The invention relates to a method and apparatus for recording depth profiles in a specimen. The specimen is illuminated with a measuring beam for recording a depth profile. The radiation, which is reflected from the specimen, is spatially superposed with the radiation of a reference beam. An interference pattern results from the superposition. The radiation intensity of the interference pattern is recorded with a detector and is supplied to an evaluation unit.
Apparatus and methods are known which include a Michelson interferometer having a measuring branch and a reference branch. In this connection, reference can be made, for example, to the following: German patent publication 3,201,801; published international patent application WO 92/19930; and, the article of D. Huang et al entitled xe2x80x9cOptical Coherence Tomographyxe2x80x9d and published in Science, Volume 254, pages 1178 to 1181, of Nov. 22, 1991. In the above-mentioned Michelson interferometers, the radiation of the measuring branch, which is reflected from the specimen, is superposed with the radiation of the reference branch. The path, which the radiation traverses in the reference branch, is modulated. Light sources, which are incoherent over time and which are multispectral, are provided as light sources and include, for example, incandescent lamps, gas-discharge lamps and laser diodes. With the superposition of the radiation, interference phenomena only occur for identical optical path lengths of reference branch and measuring branch. A profile of the specimen is recorded because of the variation of the optical path length in the reference branch. This is so because only the radiation, which is reflected in the corresponding depth of the specimen, interferes with the radiation of the reference branch.
It is disadvantageous in these methods that a movable element is required in the reference branch for the modulation of the optical path. The time span, which is required for recording a depth profile, is given by the modulation velocity. In this way, the time, which is required to record a depth profile, is dependent upon the scanning velocity of the moved element. A movement of the specimen should be prevented during a recording of a depth profile because otherwise, the allocation of the measuring point of the depth profile to the corresponding measuring point in the specimen is only possible while considering the movement of the specimen. However, a three-dimensional recording of a specimen is thereby almost impossible insofar as the movement the specimen is not controllable. For this reason, recordations of a living specimen can only be carried out to a limited extent or with limited accuracy. The required illumination time is of critical significance especially for living specimens.
It is known to measure depth profiles of a specimen in that a very broadband source is utilized as a light source as disclosed in the article of M. Bail entitled xe2x80x9cFast Optical Analysis in Volume Scatterers by Short Coherence Interferometryxe2x80x9d published in SPIE, Volume 2925, pages 298 to 303, 1996. Here, the radiation, which emanates from the light source, is split by a beam splitter into a measurement beam and into a reference beam. The radiation of the reference branch traverses different optical paths in the transmission through a greatly dispersive medium arranged in the reference branch. The optical paths are different in dependence upon the particular wavelength in the reference branch. Thereafter, the radiation of the reference path is superposed with the radiation reflected from the specimen. The beam formed in this manner is spatially split according to wavelengths by a prism mounted in the beam path. A conclusion can be drawn as to the reflectivity at the corresponding depths of the specimen from the detection of the intensity in dependence upon the wavelength assigned to this position.
There are specimens wherein the reflectivity is greatly dependent upon the wavelength of the in-radiating light. In these specimens, it is disadvantageous in this method that an is investigation in profile depths (which have a wavelength assigned thereto which exhibits only a negligible reflectivity referred to the medium to be investigated) is not possible in this profile depth. Further, the maximum scanning depth or profile depth is limited by the strongly dispersive medium and especially the dispersivity of the medium. From this results that it can be necessary that the reference beam must traverse long paths in the dispersive medium in order to achieve an acceptable scanning range via a corresponding splitting of the radiation in dependence upon the wavelength in the reference branch.
In addition to the foregoing, providing a corresponding broadband light source is only possible to a limited extent.
It is an object of the invention to provide a method and an apparatus wherein the measurement of depth profiles by means of very short light pulses is possible. It is another object of the invention to provide an apparatus and method for measuring depth profiles wherein no movable elements are needed in the reference branch.
The arrangement of the invention is for recording a depth profile in a specimen. The arrangement includes: an optical device for generating a measuring beam directed onto and reflected from the specimen and for generating a reference beam coherent to the measuring beam in respect to a reference point of time; a detector including a sensor device defining a sensor surface; a first optic for conducting the measuring beam reflected from the specimen along a first path to irradiate at least a first portion of the sensor surface; a second optic for conducting the reference beam along a second path to simultaneously irradiate at least a second portion of the sensor surface; the first and second optics being spaced from each other so as to cause the first and second portions to overlap to provide an interference effect representative of the depth profile; and, an evaluation unit connected to the detector for evaluating the interference effect.
According to a feature of the invention, a reference beam and a measuring beam are arranged so as to be spaced from each other. With this feature, a sensor surface, which is assigned to a detector, is irradiated on a subregion thereof simultaneously by the reference beam and the measuring beam. In this way, a method and an apparatus are provided with which the recording of depth profiles is possible with very short exposure times. The overlapping region defines a depth profile. In this method and apparatus, no movable parts are required for modulating the optical path in the reference branch because different optical paths of the respective radiation are traversed to the sensor surface because of the irradiation over an area referred to a beam output assigned to the particular beam. The measuring point in the corresponding depth of the specimen results from the difference of the different paths for the radiation of the reference branch and of the measuring beam to the sensor surface. In this way, a constructive interference of the corresponding radiation results.
The above subregion is simultaneously irradiated by the reference beam and the measuring beam. The interference phenomena occurring on this subregion provide information as to the reflectivities of the specimen and its nature at the corresponding depths.
It has been shown to be advantageous to assign an optic to the measuring beam and the reference beam by means of which the particular beam is expanded so that the sensor surface is irradiated by each beam over the largest possible area. The maximum scanning depth in the specimen is fixed in dependence upon the maximum difference of the optical paths from the beam output of the respective beam up reaching the sensor surface.
It has also been shown to be advantageous to guide the measuring beam and the reference beam in fibers or light conductors (preferably monomode fibers). Each of the fibers are provided at their respective ends with an optic for expanding the particular beam. The light conductors are each provided with an optic for making available a high aperture and/or for expanding the beam. Such light conductors are already available as cost-effective standard components.
In some embodiments, it has been shown to be advantageous to provide a single optic for the expansion of the measurement and reference beams. If the sensor surface is configured in the form of a sensor line (that is, a plurality of sensors arranged along a line), it has been shown to be advantageous to provide an optic for focusing the radiation of the measuring beam and the reference beam on this sensor line. An expansion of the respective beams in the direction in which the sensor line extends is retained so that the largest possible component region is irradiated simultaneously by the measurement and reference beams.
Providing a sensor line is advantageous for reducing costs because all data of a depth profile are obtained by a single sensor line.